As fluorine-containing copolymers are well known copolymers of fluoroolefins such as tetrafluoroethylene (TFE) and chlorotrifluoroethylene (CTFE) and straight or branched chain olefins such as ethylene, propylene and alkyl vinyl ether. Since these fluorine-containing copolymers can be subjected to melt molding such as extrusion molding and injection molding and have excellent heat resistance, water resistance, weather resistance and like properties which are peculiar to fluororesins, they are useful as materials for container, heat or electric insulator, weather resistant film, etc.
Among the above fluorine-containing copolymers, copolymers of ethylene (E) and fluoroolefin are especially known as ETFE (ethylene-tetrafluoroethylene copolymer), ECTFE (ethylene-chlorotrifluoroethylene copolymer), etc. However, these copolymers are not applicable to optical materials since they are crystallizable and thus low in transparency. The copolymers are also not applicable to coating materials because of insolubility in organic solvents.
Further, copolymers of propylene, alkyl vinyl ether or the like and fluoroolefin are well dissolved in organic solvents. However, their low glass transition point of below 50.degree. C. requires crosslinking before use, if heat resistance is required.
In recent years, there are known polymers which have cyclic structures in the main chains prepared by cyclic polymerization of perfluoroether (Unexamined Japanese Patent Publication No. 238111/1988). However, these copolymers are expensive since the monomers to be used are difficult to prepare. The copolymers are also costly and not preferable, because only special fluorinecontaining compounds can be used for solvents when the copolymers are applied to coating.